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Descriptions, Definitions, Recommendations

Types
of Lenses

Referenced
below is some helpful technical information including lens types
and terminology definitions. Use these guides in addition to the
magnifiers recommendations below to help you choose the correct
magnifier.

Simple
Positive Lens - A single piece of optical glass or acrylic with
two convex surfaces (curved outward). Designed for low magnification.

Simple
Negative Lens - A single piece of optical glass or acrylic with
two concave surfaces (curved inward). Used in conjunction with positive
lenses.

Achromatic
Lens - A positive simple lens cemented to a negative simple
lens. Achromatic lenses are corrected for two colors and produce
flatter fields of view at higher powers.

Double
Lens - Two simple lenses used as a system but not cemented together.
This system produces a far superior image than a simple lens.

Technical
Definitions

Power: The number of times a lens or lenses multiplies an image with respect
to the original size. Low power is recommended for scanning large
surfaces. Higher powers are generally used for inspecting small
areas.

Field
of View: The maximum area seen through a magnifier. The entire
field of view may not be in focus. See Flatness of Field.

Flatness
of Field: Due to the physical laws of optics, the outer part
of the image formed by a lens may be out of focus. The greater the
power and curvature of the lens, the more pronounced this problem
becomes. Magnifiers with multiple lenses can reduce this problem.
The visible area which appears in focus is the flat field.

Working
Distance: The distance from the magnifier lens to the object
being viewed. As magnification power increases, the working distance
decreases.

Aberration: This is a distortion caused by the lens. Spherical aberration
is distortion of the image, as explained in Flatness of Field. Chromatic
aberration is distortion of colors being viewed, as explained in
Color Distortion.

Color
Distortion: Lenses produce a prism effect which causes the image
to develop false color fringes. This is due to the fact that different
colors focus at different points. Achromatic lenses correct this
problem by focusing many colors at the same point.

Parallax: Parallax is the apparent misalignment of two different items
when viewed from an angle. It is the cause of improper registration
when stripping multiple color jobs. Parallax is eliminated when
viewing straight down on registration marks instead of from a slight
angle.

Coated
Lens: A coated lens helps to minimize or eliminate color distortion
caused by different colors focusing at different locations. Optimum
color viewing is obtained from coated achromatic lenses.

Magnifier
Recommendations

Magnifier

Usage

1"
x 1" Linen Tester (6x)

General
purpose work. The graphic arts most popular magnifier.

12x
Plastic Base Loupe

Excellent
working distance, wide field and clear base makes this unit
best suited for darkroom and stripping areas.

10x
& 12x Folding Loupes

Sufficient
working distance to be used in stripping department for touchups.
Achromatic lens system is perfect for press checks.

Choosing the Right Loupe for the Job, a.k.a. Not All Magnifiers
Are Created Equal
We are asked the question, “What is the best loupe?”
The answer is always the same, “For what purpose?”
This usually creates more confusion but it is easily corrected.
It is definitely subjective, but it is necessary to qualify
the usage before recommending the loupe. All references
can be seen www.peakoptics.comGENERAL INSPECTION LOUPES
Loupes that are used for general inspection purposes are
usually equipped with a larger diameter, lower magnification
lens. This combination allows the viewer to quickly scan
a large area. If a defect is noted, it is not unusual to
do a closer inspection of that specific area with a higher
power loupe. General inspection loupes are usually in the
4X to 6X range. Their most important characteristics are
a flat, bright field and a large flat field-of-view to lens
diameter ratio. PRESS-CHECK LOUPES
The most common press-check magnifier is the 1" X 1"
Linen Tester. This may be OK for small formats or single
colors to give a quick look, but for process color printing
a stronger, higher quality loupe is needed. These loupes
need to have a large, flat
field-of-view to lens diameter ratio, they should be achromatic
and color coated. The magnifications for loupes of this
type range from 10X to 20X. Several factors to remember
with these loupes are that as the magnification increases,
the lens diameter generally decreases, as the magnification
increases the working distance between the lens and the
subject decreases and as the magnification increases the
flat, field-of-view decreases. Some excellent examples of
Press-Check Loupes are Lithco Litho Mags, the Lithco 12X
Plastic Base Loupe and the Lithco 12X Swing-Base Magnifier.STRIPPING TABLE LOUPES
There are two primary types of loupes designed for the stripping
table. The first is an anti-parallax loupe and the second
may well be called a “touch-up” loupe. The first
type facilitates alignment of registration marks. The optical
design requires that you look perfectly straight down on
the marks in order to see them, thus allowing you to align
them in exact registration. Prime examples of anti-parallax
loupes are the Octagon Anti-Parallax Loupes. The main feature
of the touch-up loupe is the ability to get between the
lens and work surface to do scribing or opaquing. These
include the Lithco 12X Plastic Base Loupe 6X Wide Field
Magnifier and 5X Round Stand Magnifier.

Regardless of your magnifier needs, GWJ co (Peak
Optics) has the right product for the job.